AttachAction doesn't work

I have a simple one-to-many relation between Clients and Contacts. I want to select existing contacts for a Client from the RelationManager.

If I use the AttachAction from the Header Actions I get an error, which is available here:
https://flareapp.io/share/lm2DjkL7#F70

Call to undefined method Illuminate\Database\Eloquent\Relations\HasMany::getRelatedKeyName()

Not sure what I am doing wrong?
Flare
Call to undefined method Illuminate\Database\Eloquent\Relations\HasMany::getRelatedKeyName() - The error occurred at https://careerguide.test:8443/admin/clients/46/edit
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